The warning came in an interview with the 'Kommersant' newspaper with Moscow's representative at the United Nations, Vitaly Churkin.

Mr Churkin has been the public face of Russia's backing for Mr Assad since the start of the Syrian crisis, so his words, if they reflect Moscow's real thinking, have some weight.

"Russia has invested very seriously in this crisis, politically, diplomatically and now also militarily," he told the newspaper. "Therefore we would like Assad also to respond to this.

"If they proceed on the basis that no ceasefire is necessary and they need to fight to a victorious end, then this conflict will last a very long time and that is terrifying to imagine."

Mr Assad last week said he was determined to try to regain control over all the country - something that looks remote but less so since Russia's air campaign forced the non-Isil rebels into retreat across much of the north.

His words were seen as rejecting a ceasefire that had been negotiated between the United States, Russia and regional powers.

At the same time, regime forces, thanks to the air strikes and Iranian support, managed to cut off rebel-held parts of the city of Aleppo from the border with Turkey to the north, a key supply route.

Mr Churkin said Mr Assad now had to "follow Russia's leadership" in resolving the crisis, which it could still do in a "dignified way".

Russia has always insisted it is not "wedded" to the idea of Mr Assad staying in power, only to preserving the Syrian state.

This has repeatedly led to hopes that President Vladimir Putin is open to a deal under which Mr Assad would be replaced but Russian interests in the country, including a naval base on the Mediterranean, would be honoured.

Mr Putin has never actually proposed an alternative to Mr Assad, however.

In the light of the fighting, Staffan de Mistura, the UN envoy on Syria, said on Thursday it was unlikely peace talks would resume as planned in Geneva on February 25.

Mr Churkin's apparent reassurance that Russia will not back Mr Assad in a fight to the death will be of little comfort to the rebels.

They are now facing a powerful new enemy - one backed by the United States, which they had been hoping would come to their rescue.

The Kurdish YPG militia, fighting under a new flag of convenience as the Syrian Democratic Forces, has pushed back the rebels north-west of Aleppo, some of their heartland territory.

The YPG, an offshoot of the Turkish guerrilla organisation the PKK, has been shelled by Turkish forces keen to stop them getting an even greater foothold.

But the YPG on Thursday said it was making good ground on its eastern front against Isil thanks to air support from the US.

Turkey said the US was sending "mixed messages" over the YPG.

In another example, the Obama administration said it "had not determined" who carried out a suicide bomb attack on a Turkish military convoy in Ankara that killed 28 people on Wednesday.